(1) Field of the Invention
The present invention relates to an improved method for preparing ethylene polymers. More particularly, the present invention relates to an industrially advantageous method for preparing, homopolymers of ethylene or copolymers of ethylene and one or more .alpha.-olefins other than ethylene by using a specified Phillips-type catalyst having a particularly high catalytic activity for the polymerization and a very short induction period of the polymerization.
(2) Description of the Related Art
There are many methods or processes for the polymerization of ethylene; i.e., high pressure polymerization processes, medium pressure polymerization processes, low pressure polymerization processes, radiation polymerization methods, etc. It is well known that Phillips catalysts or Standard Oil catalysts are employed for the medium pressure processes, while Ziegler catalysts are used for the low pressure processes. The Phillips catalysts used in the medium pressure processes are each usually prepared by depositing a chromium compound such as chromium oxide, onto an inorganic oxide support such as silica, silica-alumina, and then calcining the resulting solid in air. A solvent having a suitable boiling point is usually used in the polymerization of ethylene over these catalysts.
As a matter of fact, such polymerization processes in which the Phillips catalysts are employed have such disadvantages as follows:
The averaged molecular weight of the polymers obtained as well as the activity of the catalysts used in the processes greatly depends on the temperature of the polymerization; therefore, it is generally necessary to conduct the polymerization at a temperature of about 100.degree. to 200.degree. C. in order to attain a sufficiently high catalytic activity for the purpose of the production of polyethylenes having an averaged molecular weight of about tens of thousands to hundreds of thousands and being useful in a variety of fields of utilization. However, the polymerization in such a temperature range makes it difficult to increase the concentration of the polymers in the product mixture to a value of not less than about 20% by weight, since the dissolution of the polymers produced into the solvent used causes a sharp increase in the viscosity of the product solution as a result. It has, therefore, been required to develop a suitable catalyst having a sufficiently high catalytic activity especially at temperatures of not higher than about 105.degree. C. because, at such low temperatures, the polymerization can be effectively operated by means of a so-called slurry polymerization technique.
Accordingly, with the intention of improving the activity of Phillips catalysts, many catalysts including those that can be obtained by the combination of Phillips catalysts with organoaluminium compounds, organozinc compounds or the like have been developed (Japanese Patent publication Nos. 22144/1961, 27415/1968, 34759/1974, Japanese Patent Laid-open No. 101477/1973, etc.).
Nevertheless, the activities of these catalysts for the polymerization are still insufficient.
Some catalysts obtained by the modification of Phillips catalysts with organomagnesium compounds are disclosed in Japanese Patent Laid-open No. 120713/1981; however, these catalysts have disadvantages in that they show a long induction period of the polymerization and have a low stability in the polymerization, although they have improved catalytic activities for the polymerization.
Furthermore, a process for the polymerization of ethylene characterized by conducting the prepolymerization of ethylene by using a Phillips catalyst and then polymerizing further ethylene in the presence of the resulting reaction mixture comprising of the catalyst used and the prepolymer produced is disclosed in Japanese Patent Laid-open No. 106610/1986. However, this process has such a disadvantage in that the catalytic activity is still insufficient, although the induction period of the polymerization is rather short.